No you can not just change the breaker. A 2 pole 40 amp does not mean 80 amps. You need a 2 pole 80 amp breaker. Also the size of your wire will need to change. If i can remember right you will need a size 6 awg wire for 80 amps.

The rating of your "breaker box" refers only to the maximum ampacity of that box, not the size of your service. If this is a main panel, your service is most likely 100 amps.

You are correct in noting that it isn't a matter of adding up the nominal sizes of existing breakers--it takes a demand load analysis to determine the adequacy of a service.

However, it seems unlikely that you can safely add an 80 amp load to a 100A main panel that already feeds central air conditioning (say, 40 amps?), an electric oven/range (again, 40 amps or possibly 50), kitchen counter appliances, laundry and refrigerator as well as the lighting and general purpose circuits of your house. Without assessing the loads, that screams "overload" to me.

In order to add an 80A 240V circuit, you'd almost certainly need to upgrade your entire electrical service. And, no, you can't just swap out breakers.

My advice? Consult an electrician to evaluate your loads and capacity before adding this circuit.

If i can remember right you will need a size 6 awg wire for 80 amps.

No way! #6 awg copper wire is rated for a non-continuous branch circuit load of up to 55 amps, although it can be protected by rule with a 60 amp breaker.

#4AWG copper can be used for 80A circuits under some circumstances in some jurisdictions. This particular code interpretation is one that varies from place to place.

#3AWG copper or #2AWG aluminum appear to be the minimal safe choice here but check with your AHJ to be sure of the code requirements in your area. (I'm also assuming that your proposed tankless heater does not have to be treated as a "continuous load", as is the case with storage-tank heaters, but check the manufacturer's specifications to be sure.)

p.s. A two-pole 40A breaker does NOT provide for 80 amps. It protects a circuit for up to 40 amps at 240V. Period.

Electric tankless units are typically fed with TWO (sometimes THREE) 240V circuits (not just two 120V legs). For instance, the 80Amp unit I'm looking at here has two 50Amp circuits feed You'd run two 8-2 cables to it.

terribletom is correct it needs to be #4 AWG. THHN is rated for 75 amps for a #4 AWG. #3 is an over kill and it will cost more. THHN #3 is rated for 110 amps.

The doesn't want an 80 Amp circuit.
He wants two 40 AMP 240 circuits.

lblake321, absolutely forget one of these units with a 100A service, which is what you have.

If you pushed the limit of your service any further, you would have to take a shower with a flashlight. Don't forget to turn the tv off or you might only get a hot shower for 5 minutes or so before the breaker trips.

Before it is installed you should really think about your actual usage demands. Say you have two baths and a 40 gallon water heater.. how often do two people take baths or showers at the same time and are able to have the amount of hot water they want... If not and I know it would be nice to have that ability, why? Say you put a unit large enough to support one bathroom with a heat rise high enough you will have good hot water winter or summer. When you are done with your bath or shower 5 mins or 50 later tell the other person to hop in.

Many of these are sized way over the top of what we as families are capable of enjoying with our tanked units and most utilities do not care to upgrade for them because it is rare that a family runs a load of whites on hot in the washer, does a load of dishes and both baths.

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How many KW's hypothetically are used if both showers were ran one night for approx 30 mins while a load of laundry on hot was going and the dishwasher compared to the washer going early in the evening and the dishwasher running after dinner and later two 30min showers are taken back to back.

WOW a peak load that is killer in a 30 minute period a night compared to a much lower load spread out over a much longer period.

Ok now everyone on your utility circuit has instant on and decides to do both methods one week everything at once and one week like they have tanked units, oh yea its August and every air conditioner in town has been running all day and most still are.

I really like 'green' aspect of instant on heaters.. but if they are all sized for all or even 3/4 of the actual gallon per min load possible and as we get busier and busier in our lives and start making use of the capability for all hot water to be consumed at once or even 3/4 the KW usage per family will not jump so much because of the 100% efficiency but the load capacity to support this will be tremendously higher than spreading it out.

not meaning to rant.. and not bashing gas instant on.. or any instant on, just that it seems some are sized waay wrong.